In multiple level interconnect structures in semiconductor processing, one of the major challenges is the dimensional control of the conductive interconnect features (the line width and height), which is critical to achieve necessary circuit performance of the device. To achieve optimum device performance, there is limited tolerance of the profile variation in interconnect structures. This dimensional control requirement demands metrology solutions to characterize the interconnect structures at all metal levels.
In one conventional metrology technique, a single measurement of the sample is made. The sample is modeled mathematically and the mathematically predicted data is compared to the measurement data. When a good fit occurs, the model is said to accurately describe the sample. The model may be repeatedly adjusted until the fit is considered to be within tolerance. In some systems, multiple varying models are generated and stored, along with their associated mathematically predicted data, in a library that is consulted during measurement of a sample.
Modeling techniques are particularly useful when the sample is a simple structure, such as uniform films. Unfortunately, when the sample is complicated, such as overlying orthogonally arranged periodic patterns, analytically modeling the sample can be difficult. For example, the test structure for a copper interconnect usually features stacked copper gratings with alternating orientations and different line pitches. Metrology solutions using scatterometry based techniques require 3D modeling for these structures, which are often impractical due to the structure complexity, large parameter space and serious parameter correlations.
Accordingly, what is needed is an improved optical metrology process that can be used to measure complicated sample structures.